Process for producing multimodal polyethylene blends including ultra-high molecular weight components

ABSTRACT

The present application relates to a process for producing a multimodal polyethylene composition comprising the steps of at least partially melting a first polyethylene resin (A) having a viscosity average molecular weight My of equal to or more than 700 kg/mol to equal to or less than 10,000 kg/mol and a density of equal to or more than 920 kg/m 3  to equal to or less than 960 kg/m 3  in a first homogenizing device, at least partially melting a second polyethylene resin (B) having a Mw of equal to or more than 50 kg/mol to less than 700 kg/mol, and a density of equal to or more than 910 kg/m 3  to equal to or less than 960 kg/m 3  in a second homogenizing device, combining the at least partially molten first polyethylene resin (A) with the at least partially molten second polyethylene resin (B) in said second homogenizing device, compounding the combined first polyethylene resin (A) and second polyethylene resin (B) in said second homogenizing device to form a multimodal polyethylene composition, wherein the multimodal polyethylene composition has a melt flow rate MFR 5  (190° C., 5 kg) of 0.01 to 10.0 g/10 min and a density of equal to or more than 910 kg/m 3  to equal to or less than 970 kg/m 3  and a polyethylene composition obtainable by said process.

The present application relates to a process for producing a multimodalpolyethylene composition with an ultra-high molecular weight componentand a conventional polyethylene component by separately melting theultra-high molecular weight component and the conventional polyethylenecomponent and combining the melts. The resultant multimodal polyethylenecomposition shows an improved homogeneity at a minimum degradationduring melt-blending.

BACKGROUND OF THE INVENTION

In multimodal polyethylene resins the high molecular weight componentusually with comonomer incorporation is responsible for the strength,including long term strength and how well this fraction is incorporatedin the total polymer mass is the key for the final product properties,in particular for high-strength and high performance products such asPE100 or PE100+ pipe resins, high-end film resins or blow mouldingresins.

Multimodal polyolefins, especially multimodal polyethylenes areinherently difficult to homogenize due to a large difference inviscosities and a large difference in particle size of the variousreactor powder particles. Especially in sequential polymerizationprocesses the high molecular weight and high viscous powder particlesare normally considerably larger than the lower molecular weightparticles. During the homogenization step the particles of highmolecular weight polyethylene are difficult to homogenize within thepolymer melt so that so-called “white spots” occur in the compoundedmaterial. These white spots usually have a size of below 10 to about 50micrometer, even though they occasionally can have a size of up to 0.1mm or even 0.5 mm, and consist of high molecular weight polymerparticles that have not been adequately dispersed in the composition.Further, when compounding polymer compositions e.g. for the productionof films gel particles with a size of about 0.01 to 1 mm often occur.These gel particles also consist of high molecular weight polymerparticles not adequately dispersed and appear as disfiguringinhomogeneities in the finished film. Still further, inhomogeneities inmultimodal polymer compositions may also increase roughness of thesurface of articles produced thereof.

One possibility to break up these high viscous particles is to usehigher shear forces during compounding. Higher shear forces are usuallyapplied to the extent which is necessary with regard to the needs, thedegradation limits of the polymer, energy costs, costs for necessaryprocess stabilizers and other physical limits such as low viscositiesand lower shear forces due to high temperatures generated and limitedcooling capacity.

High shear forces, however, applied as shear flow which is thepredominant flow in extruders and mixers, are in most cases notsufficient to break up high molecular weight polymer particles inmultimodal polyethylene resins with large viscosity differences betweenthe polymer fractions.

These compatibility problems particularly apply in the case an ultrahigh molecular weight component (UHMW) is included into a polyethylenecomposition for further improving strength properties, as it becomesmore and more difficult to homogenize the ultra high molecular weightparticles into the polymer matrix.

Thus, there is still a need for methods to incorporate ultra highmolecular weight components into multimodal polyethylene resins such asthat a homogeneous blend with a minimum of UHMW particles, so-calledwhite spots is obtained at a minimum degradation of the polymer chains.

It has surprisingly been found that this object can be achieved whenblending the UHMW polyethylene component with a polyethylene resin oflower weight average molecular weight which are both at least partiallyin liquid form to form the desired polyethylene composition. This isachieved by at least partially melting the UHMW polyethylene componentand the polyethylene resin of lower weight average molecular weightprior to the compounding step. Said composition surprisingly shows a lowamount of white spots even when blended under mild conditions in orderto avoid degradation.

SUMMARY OF THE INVENTION

The present invention is based on the surprising finding that theabove-mentioned improved homogeneity can be achieved by a process forproducing a multimodal polyethylene composition comprising the followingsteps:

-   i) at least partially melting a first polyethylene resin (A) having    a viscosity average molecular weight My of equal to or more than 700    kg/mol to equal to or less than 10,000 kg/mol and a density of equal    to or more than 920 kg/m³ to equal to or less than 960 kg/m³ in a    first homogenizing device,-   ii) at least partially melting a second polyethylene resin (B)    having a Mw of equal to or more than 50 kg/mol to less than 700    kg/mol, and a density of equal to or more than 910 kg/m³ to equal to    or less than 960 kg/m³ in a second homogenizing device,-   iii) combining the at least partially molten first polyethylene    resin (A) with the at least partially molten second polyethylene    resin (B) in said second homogenizing device,-   iv) compounding the combined first polyethylene resin (A) and second    polyethylene resin (B) in said second homogenizing device to form a    multimodal polyethylene composition,

wherein the multimodal polyethylene composition has a melt flow rateMFR₅ (190° C., 5 kg) of 0.01 to 10.0 g/10 min and a density of equal toor more than 910 kg/m³ to equal to or less than 970 kg/m³.

In a further aspect the present invention provides a polyethylenecomposition obtainable by a process comprising the following steps:

-   i) at least partially melting a first polyethylene resin (A) having    a viscosity average molecular weight My of equal to or more than 700    kg/mol to equal to or less than 10,000 kg/mol and a density of equal    to or more than 920 kg/m³ to equal to or less than 960 kg/m³ in a    first homogenizing device,-   ii) at least partially melting a second polyethylene resin (B)    having a Mw of equal to or more than 50 kg/mol to less than 700    kg/mol, and a density of equal to or more than 910 kg/m³ to equal to    or less than 960 kg/m³ in a second homogenizing device,-   iii) combining the at least partially molten first polyethylene    resin (A) with the at least partially molten second polyethylene    resin (B) in said second homogenizing device,-   iv) compounding the combined first polyethylene resin (A) and second    polyethylene resin (B) in said second homogenizing device to form a    multimodal polyethylene composition,

wherein the multimodal polyethylene composition has a melt flow rateMFR₅ (190° C., 5 kg) of 0.01 to 10.0 g/10 min and a density of equal toor more than 910 kg/m³ to equal to or less than 970 kg/m³.

DETAILED DESCRIPTION OF THE INVENTION Definitions

A polyethylene composition according to the present invention denotes apolymer derived from at least 50 mol-% ethylene monomer units andadditional comonomer units.

The term ‘homopolymer’ thereby denotes a polymer consisting essentiallyof ethylene monomer units. Due to the requirements of large-scalepolymerization it may be possible that the ethylene homopolymer includesminor amounts of additional comonomer units, which usually are below0.05 mol %, preferably below 0.01 mol % of the ethylene homopolymer.Accordingly, the term ‘copolymer’ denotes a polymer derived fromethylene monomer units and additional comonomer units in an amount ofmore than 0.05 mol %.

Usually, a polyethylene composition comprising at least two polyethylenefractions, which have been produced under different polymerisationconditions resulting in different (weight average) molecular weights forthe fractions, is referred to as “multimodal”. The prefix “multi”relates to the number of different polymer fractions of the compositionconsists of. Thus, for example, a composition consisting of twofractions only is called “bimodal”, whereas a composition consisting ofthree fractions is called “trimodal”.

The ultra-high molecular weight (UHMW) component in the polyethylenecomposition is the component having a viscosity average molecular weightMv of 700 kg/mol to 10,000 kg/mol. Thus, in the wording of claim 1 theUHMW component denotes the first polyethylene resin (A) having aviscosity average molecular weight Mv of equal to or more than 700kg/mol to equal to or less than 10,000 kg/mol.

For determining the molecular weight of a polyolefin resin or componentseveral statistical methods are known in the art. In practice fouraverages are used, representing the weighted mean taken with the molefraction, the weight fraction, and two other functions which can berelated to measured quantities:

-   -   Number average molar mass or Mn (also loosely referred to as        Number Average Molecular Weight) with Mn=ΣM_(i)N_(i)/ΣN_(i)    -   Mass average molar mass or Mw (w is for weight; also commonly        referred to as weight average) with Mw=ΣM_(i) ²N_(i)/ΣM_(i)N_(i)    -   Z average molar mass or Mz with Mz=ΣM_(i) ³N_(i)/ΣM_(i) ²N_(i)    -   Viscosity average molar mass or Mv with Mv=[ΣM_(i)        ^(1+a)N_(i)/ΣM_(i)N_(i)]^(1a)

Here a is the exponent in the Mark-Houwink equation that relates theintrinsic viscosity to molar mass.

The term ‘base resin’ denotes the polymeric part of the compositionwithout fillers such as carbon black. A person skilled in the art willunderstand that the measurements as to the base resin require thepresence of stabilizers.

In addition to the base resin, usual additives for utilization withpolyolefins, such as pigments (e.g. carbon black), stabilizers (e.g.antioxidant agents), antacids and/or anti-UV's, antistatic agents andutilization agents (such as processing aid agents) may be present in thepolyethylene composition. Preferably, the amount of these additives is10 wt % or below, more preferably 8 wt % or below, most preferably 5 wt% or below, of the composition. Further, the composition may comprisefillers, like mineral fillers and modifiers, in an amount of up to 20%by weight of the composition, preferably up to 10% by weight of thecomposition, provided that such fillers have no negative impact on theproperties of the composition. Preferably, however, such mineral fillersare not present. For the purpose of the determination of the homogeneityof the polyethylene composition by means of the white spot area and theISO rating as described in the example section the polyethylenecomposition does not comprise any mineral fillers as these fillers wouldimpair the measurement.

Preferably, the composition comprises carbon black in an amount of 8 wt% or below, more preferably in an amount of 1 to 4 wt %, of the totalcomposition.

Further preferred, the amount of additives different from carbon blackand the optional fillers is 1 wt % or less, more preferably 0.5 wt % orless.

The term “homogenization device” denotes a device suitable forhomogenizing polyolefin melts or at least partially molten polyolefinresins by distributive mixing. Suitable homogenization devices aremixers, kneaders and extruders. These devices are known in the art.

Extruders can be classified as small extruders and large extruders. Anextruder is denoted as small if the temperature of the melt in theextruder effectively could be influenced by the extruder barreltemperatures by heat conduction, i.e. by external heating or cooling ofthe barrel.

The term “blending” denotes any method for admixing components of thepolyethylene composition such as polyethylene components and additives.

Suitable blending methods are in situ blending, such as polymerizing onepolyethylene compound in the presence of another polyethylene compound,or mechanical blending, such as dry blending of solid compounds such aspowders and/or pellets, melt blending of liquid compounds such as meltsof polyethylene compounds or blending of a liquid compound with a solidcompound. Devices for these blending methods are known in the art.

General

Process

First Homogenizing Step

In the process according to the invention the first polyethylene resin(A) is homogenized by at least partially melting the first polyethyleneresin (A) in a first homogenizing device.

Thereby, polyethylene resin (A) can be fed to the first homogenizingdevice as pellets or as powder. Preferably, polyethylene resin (A) isintroduced as powder.

Suitable first homogenization devices are mixers or extruders.Especially suitable are twin screw extruders such as e.g. Coperion ZSKtwin screw extruders.

The set point for the barrel temperature in the extruder is preferablyfrom 150° C. to 220° C., more preferably from 160° C. to 210° C. andmost preferably from 170° C. to 200° C. For small extruders and also forlarge extruders during the start-up the barrels are typically heated,for instance, by electric bands. However, as it is well understood bythe person skilled in the art large extruders generally operateadiabatically and then the barrel temperatures are not controlled andpractically linked to the temperatures generated in the melt along thelength of the extruder.

The throughput is selected based on the desired production volume. Asthe person skilled in the art understands greater throughput can beachieved by extruders having a greater diameter. Useful scale-upprinciples for mixing is presented, among others, in Rauwendaal, PolymerExtrusion, Hanser Publishers, Munich, 1986 (ISBN 3-446-14196-0), inTable 8-4 on page 439. It shows that the ratio of the output rates isdirectly proportional to the cube of the ratio of the diameters.

${\overset{.}{V}}_{2} = {{\overset{.}{V}}_{1} \cdot ( \frac{D_{2}}{D_{1}} )^{3}}$

where V₂ and D₂ are the output rate and diameter of the large extruderand V₁ and D₁ are output rate and the diameter of the small extruder.

For small laboratory and pilot scale extruders throughput within a rangeof from about 1 up to about 200 kg/h would be appropriate and for largeproduction scale extruders the throughput could be from about 300 up toabout 20,000 kg/h.

The screw speed is preferably from 300 min⁻¹ to 800 min⁻¹, morepreferably 400 min⁻¹ to 600 min⁻¹.

For a co-rotating twin screw extruder, the feed port of the firsthomogenisation device is preferably located so that the L/D is from 12to 20, more preferably from 14 to 18, wherein L is the distance from thefeed port to the die and D is the diameter.

It is possible that the first homogenization device has an additionalfeed port downstream of the above-mentioned feed port. It is thenpreferred that the distance between such additional feed port and thedie would be such that the ratio of said distance to the screw diameterwould not be less than 10.

It is, however, preferred to feed all the components to the firsthomogenisation device via one feed port. For counter-rotating twin screwextruders/mixers, which normally are shorter in length compared toco-rotating twin screw extruders, e.g. Kobe LCM and Japan Steel WorksCIM, all the components are preferably feed via one feed port.

To either one or both feeding zone additives as defined above can beadded.

In the first homogenization device the first polyethylene resin (A) isat least partially melted. It is preferred that at least 20 wt %, morepreferably at least 50 wt %, still more preferably at least 75 wt % andmost preferably at least 95 wt % of the first polyethylene resin (A) ismelted when exiting the first homogenization device. In a preferredembodiment 100 wt % of the first polyethylene resin (A) is melted whenexiting the first homogenization device. Usually, in a large extruderthe 100 wt % of the first polyethylene resin (A) is melted when exitingthe first homogenization device.

The at least partially melted polyethylene resin (A) exiting the firsthomogenization device is preferably introduced into the secondhomogenization device without solidification or pelletizing.

It is preferred to introduce polyethylene resin (C) also to the firsthomogenisation device. Then, the polyethylene resins (A) and (C) may beintroduced through the same feed port or through different feed ports.It is preferred to introduce both polyethylene resins (A) and (C)through the same feed port.

Second Homogenization Step

In the process according to the invention the second polyethylene resin(B) is homogenized by at least partially melting the second polyethyleneresin (B) in a second homogenizing device.

Preferably the second homogenisation device has a diameter which isequal to or greater than the diameter of the first homogenisationdevice. Especially preferably the second homogenisation device has agreater diameter than the first homogenisation device.

Thereby, polyethylene resin (B) can be fed to the second homogenizingdevice as pellets or as powder. Where large extruders are used near thepolymer production facilities it is usually more convenient to feed thepolyethylene resin (B) as powder. On the other hand, for small extrudersor where the extruder is located far away from the production facilitiesit may be more convenient to feed the polyethylene resin (B) as pellets.

Preferably, the second homogenizing device has at least two feedingzones. Preferably a second feeding zone is situated downstream of thefirst feeding zone. It is particularly preferred to introduce thepolyethylene resin (B) into the first feeding zone. Then thepolyethylene resin (B) melts downstream the first feeding zone andpasses past the second feeding zone where the melt comprising thepolyethylene resin (A) is introduced into the second homogenisingdevice.

For a co-rotating twin screw extruder, preferably the ratio L1/D, whereL1 is the distance between the first feed port in the first feeding zoneand the die and D is the diameter of the second homogenising device, isfrom 20 to 35, more preferably from 22 to 30, Further, preferably theratio L2/D, where L2 is the distance between the second feed port in thesecond feeding zone and the die, is from 16 to 28, and more preferablyfrom 18 to 26. Furthermore, the distance between the first feed port andthe second feed port is preferably such that the ratio (L1−L2)/D is from1 to 10, and more preferably from 2 to 8, such as from 2 to 6.

For a counter-rotating twin screw extruder/mixer, the second feed portis preferably located downstream the melt valve or throttle where themelt pressure is low and upstream the second mixing section of the unit.The distance from melt throttle and the second feed port downstream ispreferably between 0 to 2 L/D and more preferably between 0 to 1 L/D.

Preferably, at least part of the polyethylene resin (B), more preferably100 wt % of the polyethylene resin (B) is introduced in the firstfeeding zone. Preferably at least part of the at least partly moltenpolyethylene resin (A), more preferably 100 wt % of the at least partlymolten polyethylene resin (A) is introduced in the second feeding zone.

To either one or both feeding zone additives as defined above can beadded.

In the second homogenization device the second polyethylene resin (B) isat least partially melted before reaching the second feeding zone. It ispreferred that at least 20 wt %, more preferably at least 50 wt %, stillmore preferably at least 75 wt % and most preferably at least 95 wt % ofthe first polyethylene resin (A) is melted before reaching the secondfeeding zone. In a preferred embodiment 100 wt % of the firstpolyethylene resin (A) is melted before reaching the second feedingzone.

Compounding Step in the Second Homogenizing Device

Preferably at the second feeding zone of the second homogenizing devicethe at least partially molten first polyethylene resin (A) is introducedto the second homogenizing device and is combined with the at leastpartially molten second polyethylene resin (B).

Preferably downstream said second feeding zone the combined firstpolyethylene resin (A) and second polyethylene resin (B) are compoundedto form a multimodal polyethylene composition.

Suitable second homogenizing devices are mixers or extruders. Especiallysuitable are twin screw extruders such as e.g. ZSK twin screw extrudersby Coperion, CIMP and TEX twin screw extruders by Japan Steel Works andKobe LCM and KTX twin screw extruders. For each type of extruder thediameter can be chosen based on the desired production rate using theprinciples discussed above. The diameter is usually indicated in theextruder name, such as ZSK40 (40 mm extruder), ZSK80 (80 mm extruder)CIMP-90 (90 mm extruder).

The set point for the barrel temperature in the extruder is preferablyfrom 150° C. to 250° C., more preferably from 170° C. to 230° C. andmost preferably from 200° C. to 220° C. The temperature settings arenormally selected to be close to the temperatures which will begenerated in the extruder in the melt by friction via the screwrotation. For small extruders and also for large extruders during thestart-up the barrels are typically heated, for instance, by electricbands. However, as it is well understood by the person skilled in theart large extruders generally operate adiabatically and then the barreltemperatures are not controlled.

The screw speed in the co-rotating twin screw extruder is preferablyfrom 100 min⁻¹ to 220 min⁻¹, more preferably 120 min⁻¹ to 180 min⁻¹. Forthe counter-rotating twin screw mixer the screw speed is normallyhigher, preferably from 200 min⁻¹ to 550 min⁻¹, more preferably from 260min⁻¹ to 500 min⁻¹. The screw speed is often related to the throughputin kg/hour, of the extruder, e.g. for a large two-speed extruder, thehigher speed is chosen when target is to produce close to the nameplatecapacity and the lower speed often chosen when milder conditions and/orlower throughputs are targeted. With a variable speed drive theextrusion conditions are easier to tailor for the particular throughputand homogenisation conditions required.

Again, the throughput is set by the desired production volume. Forscale-up purposes the discussion given for the first homogenisationdevice is valid also for the second homogenisation device.

Preferably the polyethylene composition exiting the second homogenizingdevice is pelletized before storage.

Blending of First Polyethylene Resin (A) and Third Polyethylene Resin(C)

In an especially preferred embodiment of the process according to theinvention the polyethylene resin (A) is introduced into the firsthomogenising device together with polyethylene resin (C).

Thereby, the first polyethylene resin (A) is blended with a thirdpolyethylene resin (C) to form a blend of polyethylene resins (A) and(C) prior to introducing the first polyethylene resin (A) to the firsthomogenization device.

The weight ratio of polyethylene resin (A) to polyethylene resin (C) inthe blend of polyethylene resins (A) and (C) is from 45:55 to 80:20,more preferably from 47:53 to 75:25, most preferably from 50:50 to70:30.

In this embodiment a blend of polyethylene resins (A) and (C) is used inthe first homogenization step so that for this embodiment thedescription of the first homogenization step above reads “blend ofpolyethylene resins (A) and (C)” instead of “first polyethylene resin(A)”.

In one embodiment of the process of the invention the blend ofpolyethylene resins (A) and (C) is formed by in situ blending ofpolyethylene resins (A) and (C), preferably in two subsequent steps of amultistage polymerization process. In such a multistage process theblend of polyethylene resins (A) and (C) is formed by polymerizing,respectively copolymerizing, ethylene in a reactor cascade formed by atleast a first reactor and a second reactor, whereby preferably the firstreactor is a loop reactor and further preferably, the second reactor isa gas phase reactor. Thereby, polyethylene resins (A) and (C) arepolymerized in subsequent reactor stages wherein the fractionpolymerized in the second reactor is polymerized in the presence of thefraction polymerized in the first reactor.

However, in the preferred embodiment of the process of the invention theblend of polyethylene resins (A) and (C) is formed by melt blending ofpolyethylene resins (A) and (C).

Thereby, polyethylene resins (A) and (C) can be fed to the melt blendingdevice as pellets or as powder.

For a co-rotating twin screw extruder the feed port of the firsthomogenisation device is preferably located so that the L/D is from 12to 20, more preferably from 14 to 18, wherein L is the distance from thefeed port to the die and D is the diameter. It is possible that thefirst homogenization device has an additional feed port downstream ofthe above-mentioned feed port. It is then preferred that the distancebetween such additional feed port and the die would be such that theratio of said distance to the screw diameter D would not be less than10. It is, however, preferred to feed all the components to the firsthomogenisation device via one feed port. For a counter-rotating twinscrew extruder/mixer all the components are preferably feed via one feedport.

To either one or both feeding zone additives as defined above can beadded.

In a preferred embodiment of the process of the invention the secondhomogenizing device is the main extruder for compounding the multimodalpolyethylene composition. Thereby, said first homogenizing device ispreferably a side feed extruder having an exit which is connecteddownstream of the feeding zone of the main extruder.

The different components of the polyethylene composition according tothe invention and obtainable according to the process according to theinvention as characterized as follows:

Polyethylene Resin (A)

Polyethylene resin (A) can be either a homopolymer of ethylene or acopolymer of ethylene, preferably a homopolymer of ethylene, and ischaracterized by the following properties:

Mv

Polyethylene resin (A) has a viscosity average molecular weight Mv of700 kg/mol to 10,000 kg/mol, preferably from 700 kg/mol to 7,000 kg/mol,most preferably from 700 kg/mol to 5,000 kg/mol, determined according toASTM 4020-81.

Density

Polyethylene resin (A) has a density of equal to or more than 920.0kg/m³ and equal to or less than 960.0 kg/m³, more preferably of equal toor more than 925.0 kg/m³ and equal to or less than 950.0 kg/m³, and mostpreferably of equal to or more than 930.0 kg/m³ and equal to or lessthan 940.0 kg/m³ determined according to ISO 1183-1:2004.

Polyethylene resin (A) can comprise additives as defined above.

Polyethylene Resin (B)

Polyethylene resin (B) can be a unimodal or multimodal, such as bimodal,ethylene homo- or copolymer.

It is preferred that the polyethylene resin (B) is multimodal.

Preferably, the polyethylene resin (B) is a copolymer of ethylene withat least one alpha-olefin comonomer unit. The alpha-olefin comonomerunit preferably is selected from alpha-olefin co-monomer units with 3 to12 carbon atoms, more preferably 4 to 8 carbon atoms. Suitablealpha-olefin comonomer units are 1-butene, 1-hexene and 1-octene.Thereby, 1-butene and 1-hexene are most preferred.

Polyethylene resin (B) can comprise additives as defined above.

Mw

Polyethylene resin (B) has a weight average molecular. weight Mw of 50kg/mol to less than 700 kg/mol, preferably from 70 kg/mol to 500 kg/mol,most preferably from 100 kg/mol to 300 kg/mol, determined by GPC.

MFR₅

Polyethylene resin (B) preferably has a melt flow rate MFR₅ (190° C., 5kg) of 0.01 to 5.0 g/10 min, more preferably of 0.05 to 4.0 g/10 min,and most preferably 0.1 to 3.0 g/10 min determined according to ISO1133.

Density

Polyethylene resin (B) has a density of equal to or more than 910.0kg/m³ and equal to or less than 960.0 kg/m³, more preferably of equal toor more than 915.0 kg/m³ and equal to or less than 955.0 kg/m³, and mostpreferably of equal to or more than 920.0 kg/m³ and equal to or lessthan 950.0 kg/m³ determined according to ISO 1183-1:2004.

Polymerization

Polyethylene resin (B) is usually made by a multi-stage process, i.e. aprocess which makes use of at least two reactors, one for producing alower molecular weight component and a second for producing a highermolecular weight component. These reactors may be employed in parallel,in which case the components must be mixed after production. Morecommonly, the reactors are employed in series, such that the products ofone reactor are used as the starting material in the next reactor, e.g.one component is formed in the first reactor and the second is formed inthe second reactor in the presence of the first component. In this way,the two components are more intimately mixed, since one is formed in thepresence of the other.

The polymerization reactions used in each stage may involve conventionalethylene homo-polymerization or copolymerization reactions, e.g. gasphase, slurry phase, liquid phase polymerizations, using conventionalreactors, e.g. loop reactors, gas phase reactors, batch reactors, etc.

The polymerization may be carried out continuously or batchwise,preferably the polymerization is carried out continuously.

Known two-stage processes are for instance liquid phase-liquid phaseprocesses, gas phase-gas phase processes and liquid phase-gas phaseprocesses. It is also known that these two-stage processes can furtherbe combined with one or more additional polymerization steps selectedfrom gas phase, slurry phase or liquid phase polymerization processes.

Polyethylene resin (B) is preferably produced in a multistage process,where lower molecular weight and higher molecular weight polymers(components) are produced in different polymerization steps, in anyorder.

The polymerisation is conducted in the presence of an olefinpolymerisation catalyst. The catalyst may be any catalyst which iscapable of producing the desired ethylene polymer. Suitable catalystsare, among others, Ziegler-Natta catalysts based on a transition metal,such as titanium, zirconium and/or vanadium or metallocene catalysts orlate transition metal catalysts. Especially Ziegler-Natta catalysts andmetallocene catalysts are useful as they can produce polymers within awide range of molecular weight with a high productivity.

Suitable Ziegler-Natta catalysts preferably contain a magnesiumcompound, an aluminium compound and a titanium compound supported on aparticulate support.

Polyethylene Resin (C)

The multimodal polyethylene composition may additionally comprisepolyethylene resin (C). Polyethylene resin (C) is preferably introducedto the multimodal polyethylene composition by blending polyethyleneresin (C) with polyethylene resin (A) prior to at least partiallymelting the blended polyethylene resins (A) and (C) in the firsthomogenisation device.

The weight ratio of polyethylene resin (A) to polyethylene resin (C) inthe blend of polyethylene resins (A) and (C) is from 45:55 to 80:20,more preferably from 47:53 to 75:25, most preferably from 50:50 to70:30.

Polyethylene resin (C) can be a unimodal or multimodal, such as bimodal,ethylene homo- or copolymer.

It is preferred that the polyethylene resin (C) is multimodal.Preferably, the polyethylene resin (C) is a copolymer of ethylene withat least one alpha-olefin comonomer unit. The alpha-olefin comonomerunit preferably is selected from alpha-olefin co-monomer units with 3 to12 carbon atoms, more preferably 4 to 8 carbon atoms. Suitablealpha-olefin comonomer units are 1-butene, 1-hexene and 1-octene.Thereby, 1-butene and 1-hexene are most preferred.

Polyethylene resin (C) can comprise additives as defined above.

Mw

Polyethylene resin (C) has a weight average molecular weight Mw of 50kg/mol to less than 700 kg/mol, preferably from 70 kg/mol to 500 kg/mol,most preferably from 100 kg/mol to 300 kg/mol, determined by GPC.

MFR₅

Polyethylene resin (C) preferably has a melt flow rate MFR₅ (190° C., 5kg) of 0.01 to 5.0 g/10 min, more preferably of 0.05 to 4.0 g/10 min,and most preferably 0.1 to 3.0 g/10 min determined according to ISO1133.

Density

Polyethylene resin (C) has a density of equal to or more than 910.0kg/m³ and equal to or less than 960.0 kg/m³, more preferably of equal toor more than 915.0 kg/m³ and equal to or less than 955.0 kg/m³, and mostpreferably of equal to or more than 920.0 kg/m³ and equal to or lessthan 950.0 kg/m³ determined according to ISO 1183-1:2004.

Polymerization

Polyethylene resin (C) is usually made by a multi-stage process, i.e. aprocess which makes use of at least two reactors, one for producing alower molecular weight component and a second for producing a highermolecular weight component. These reactors may be employed in parallel,in which case the components must be mixed after production. Morecommonly, the reactors are employed in series, such that the products ofone reactor are used as the starting material in the next reactor, e.g.one component is formed in the first reactor and the second is formed inthe second reactor in the presence of the first component. In this way,the two components are more intimately mixed, since one is formed in thepresence of the other.

The polymerization reactions used in each stage may involve conventionalethylene homo-polymerization or copolymerization reactions, e.g. gasphase, slurry phase, liquid phase polymerizations, using conventionalreactors, e.g. loop reactors, gas phase reactors, batch reactors, etc.

The polymerization may be carried out continuously or batchwise,preferably the polymerization is carried out continuously.

Known two-stage processes are for instance liquid phase-liquid phaseprocesses, gas phase-gas phase processes and liquid phase-gas phaseprocesses. It is also known that these two-stage processes can furtherbe combined with one or more additional polymerization steps selectedfrom gas phase, slurry phase or liquid phase polymerization processes.

Polyethylene resin (C) is preferably produced in a multistage process,where lower molecular weight and higher molecular weight polymers(components) are produced in different polymerization steps, in anyorder.

The polymerisation is conducted in the presence of an olefinpolymerisation catalyst. The catalyst may be any catalyst which iscapable of producing the desired ethylene polymer. Suitable catalystsare, among others, Ziegler-Natta catalysts based on a transition metal,such as titanium, zirconium and/or vanadium or metallocene catalysts orlate transition metal catalysts. Especially Ziegler-Natta catalysts andmetallocene catalysts are useful as they can produce polymers within awide range of molecular weight with a high productivity.

Suitable Ziegler-Natta catalysts preferably contain a magnesiumcompound, an aluminium compound and a titanium compound supported on aparticulate support.

Polyethylene resin (C) can be different from polyethylene resin (B) inat least one of the above defined properties.

In an especially preferred embodiment polyethylene resin (C) is the sameas polyethylene resin (B).

Polyethylene Composition

The multimodal polyethylene composition according to the inventioncomprises polyethylene resins (A) and (B), and optionally polyethyleneresin (C), which are blended according to the process of the invention.

Preferably, the base resin of the composition consists of thepolyethylene resins (A) and (B), and optionally polyethylene resin (C).

In one preferred embodiment, the base resin of the composition consistsof the polyethylene resins (A) and (B).

The composition can comprise additives as defined above.

The weight ratio of the first polyethylene resin (A) to the secondpolyethylene resin (B) in the polyethylene composition is preferablyfrom 0.5:95.5 to 50:50, more preferably from 3:97 to 35:65, mostpreferably from 8:92 to 30:70.

In the presence of a third polyethylene resin (C) in the polyethylenecomposition the weight ratio of the first polyethylene resin (A) to thecombined second polyethylene resin (B) and third polyethylene resin (C)in the polyethylene composition is preferably from 0.5:95.5 to 50:50,more preferably from 3:97 to 35:65, most preferably from 8:92 to 30:70.

Preferably the amount of polyethylene resin (A) in the polyethylenecomposition is 0.5 to 50 wt %, more preferably 1 to 40 wt %, morepreferably 3 to 35 wt %, still more preferably 5 to 32 wt %, and mostpreferably 8 to 30 wt % of the total polyethylene composition

The polyethylene composition is characterized by the followingproperties:

MFR₅

The composition according to the present invention preferably has a meltflow rate MFR₅ (190° C., 5 kg) of 0.01 to 10.0 g/10 min, more preferablyof 0.03 to 9.0 g/10 min, and most preferably 0.05 to 8.0 g/10 mindetermined according to ISO 1133.

MFR₂₁

The composition according to the present invention preferably has a meltflow rate MFR₂₁ (190° C., 21.6 kg) of 0.5 to 300 g/10 min, preferably of0.7 to 250 g/10 min, and most preferably of 1.0 to 200 g/10 mindetermined according to ISO 1133.

Density

The composition according to the present invention preferably has adensity of equal to or more than 910.0 kg/m³ and equal to or less than970.0 kg/m³, more preferably of equal to or more than 912.0 kg/m³ andequal to or less than 969.0 kg/m³, and most preferably of equal to ormore than 915.0 kg/m³ and equal to or less than 968.0 kg/m³ determinedaccording to ISO 1183-1:2004.

The density of the composition is influenced by the density of the baseresin and can further be adjusted by the amount of filler, usuallycarbon black, in the composition.

The density of the base resin is mainly influenced by the amount andtype of comonomer. In addition to that, the nature of the polymeroriginating mainly from the catalyst used as well as the melt flow rateplay a role. In addition to that, it should be stressed that thecomonomer does not need to be a single comonomer. Mixtures of comonomersare also possible.

The composition is further characterized by specific rheologicalproperties.

Complex Viscosity Eta_(0.1)

The composition according to the present invention preferably has acomplex viscosity determined at a frequency of 0.1 kPa, eta_(0.1), of5,000 Pa's to 100,000 Pa·s, more preferably 6,000 Pa·s to 85,000 Pa·s,even more preferably 8,000 Pa·s to 70,000 Pa·s and most preferably10,000 Pa·s to 55,000 Pa·s. For certain applications, such as HD pipe,MD pipe or injection moulding applications, eta_(0.1) can be of 5,000Pa·s to 400,000 Pa·s, preferably of 6,000 Pa·s to 350,000 Pa·s, evenmore preferably of 8,000 Pa·s to 300,000 Pa·s, and most preferably of10,000 Pa·s to 250,000 Pa·s.

Viscosities determined at a low frequency or shear rate, such aseta_(0.1), are a measure for the molecular weight of a polyethylenecomposition as they are directly proportional with the weight averagemolecular weight Mw. Thus, they can also be used as a measure for thedegradation of the polyethylene composition by comparing eta_(0.1) ofthe final composition after blending the first polyethylene resin (A)with the second polyethylene resin (B), eta_(0.1) (Composition) andeta_(0.1) of the second polyethylene resin (B) prior to blending,eta_(0.1)(B).

It is preferred that the ratio of the complex viscosity at a frequencyof 0.1 rad/s of the multimodal polyethylene composition, eta_(0.1)(Composition), to the complex viscosity determined at a frequency of 0.1rad/s of the polyethylene resin (B), eta_(0.1) (B), is in the range of0.8 to 5.0, more preferably 0.9 to 4.0, and most preferably 1.0 to 3.5.

SHI_(0.1/100)

The composition preferably has a shear thinning index SHI_(0.1/100) of 7to 30, more preferably a shear thinning index SHI_(0.1/100) of 8 to 27,even more preferably a shear thinning index SHI_(0.1/100) of 9 to 24 andmost preferably a shear thinning index SHI_(0.1/100) of 9.5 to 21.

The shear thinning index is a measure of the broadness of the molecularweight distribution of the polyethylene composition.

White Spot Area (WSA)

The polyethylene composition according to the present inventionpreferably has a white spot area of not more than 5.0%, more preferablyof not more than 4.0%, even more preferably of not more than 3.5% andmost preferably of not more than 3.0%. The lower limit of the white spotarea is usually 0.01%.

ISO Rating

The polyethylene composition according to the present inventionpreferably has an ISO rating of not more than 7.0, more preferably notmore than 6.0, even more preferably not more than 4.0, most preferablynot more than 3.5. The lower limit of the ISO rating is usually 0.1.

The white spot area test and the ISO rating are measures for thehomogeneity of a polyethylene composition. When compounding polyethylenecompositions e.g. for producing pipes, so-called “white spots” occur inthe compounded material. These white spots usually have a size of below10 to about 50 micrometer and consist of non-pigmented, high molecularweight polymer agglomerates/particles that have not been adequatelydispersed in the composition. These inhomogeneities in polymercompositions may increase roughness of the surface of articles producedthereof and impair their strength properties.

It is known that homogeneity of a multimodal polymer composition can beimproved by applying multiple compounding steps and/or particularcompounding conditions to the resin coming from the reactor. Thesemeasures, however, have the disadvantage that they are associated with asignificant increase in production costs for the composition andpossibly degradation of the polymer.

Applications

The polyethylene compositions produced according to the presentinvention are suitable for different applications such as steel pipecoating, high density (HD) pipe applications, film applications, such aslinear low density (LLD) films, medium density (MD) films and highdensity (HD) films, injection moulding applications and cable jacketing.For these different applications the polyethylene composition has thefollowing properties:

Steel Pipe Coating

For steel pipe coating the composition according to the presentinvention preferably has a melt flow rate MFR₅ (190° C., 5 kg) of 0.05to 5.0 g/10 min, more preferably of 0.1 to 2.5 g/10 min, and mostpreferably 0.2 to 1.0 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 935.0kg/m³ and equal to or less than 960.0 kg/m³, more preferably of equal toor more than 936.5 kg/m³ and equal to or less than 957.0 kg/m³, and mostpreferably of equal to or more than 938.0 kg/m³ and equal to or lessthan 955.0 kg/m³ determined according to ISO 1183-1:2004.

The composition preferably has a weight average molecular weight Mw of50 kg/mol to 300 kg/mol, more preferably 70 kg/mol to 250 kg/mol,determined by GPC.

LLD Films

For LLD films the composition according to the present inventionpreferably has a melt flow rate MFR₅ (190° C., 5 kg) of 0.5 to 5.0 g/10min, more preferably of 0.6 to 4.0 g/10 min, and most preferably 0.8 to3.0 g/10 min determined according to ISO 1133.

The composition preferably has a melt flow rate MFR₂₁ (190° C., 21.6 kg)of 10 to 100 g/10 min, preferably of 12 to 80 g/10 min, and mostpreferably of 15 to 70 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 910.0kg/m³ and equal to or less than 930.0 kg/m³, more preferably of equal toor more than 912.5 kg/m³ and equal to or less than 927.0 kg/m³, and mostpreferably of equal to or more than 915.0 kg/m³ and equal to or lessthan 925.0 kg/m³ determined according to ISO 1183-1:2004.

The composition preferably has a weight average molecular weight Mw of100 kg/mol to 350 kg/mol, more preferably 130 kg/mol to 300 kg/mol,determined by GPC.

MD Film

For MD films the composition according to the present inventionpreferably a melt flow rate MFR₂₁ (190° C., 21.6 kg) of 2.5 to 50 g/10min, preferably of 3.5 to 40 g/10 min, and most preferably of 5 to 30g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 925.0kg/m³ and equal to or less than 945.0 kg/m³, more preferably of equal toor more than 927.5 kg/m³ and equal to or less than 943.0 kg/m³, and mostpreferably of equal to or more than 930.0 kg/m³ and equal to or lessthan 940.0 kg/m³ determined according to ISO 1183-1:2004.

HD Films

For HD films the composition according to the present inventionpreferably a melt flow rate MFR₂₁ (190° C., 21.6 kg) of 2.5 to 20 g/10min, preferably of 3 to 15 g/10 min, and most preferably of 4 to 10 g/10min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 940.0kg/m³ and equal to or less than 970.0 kg/m³, more preferably of equal toor more than 942.0 kg/m³ and equal to or less than 965.0 kg/m³, and mostpreferably of equal to or more than 945.0 kg/m³ and equal to or lessthan 960.0 kg/m³ determined according to ISO 1183-1:2004.

Injection Moulding

For injection moulding the composition according to the presentinvention preferably has a melt flow rate MFR₂ (190° C., 2.16 kg) of 0.2to 4.0 g/10 min, preferably of 0.4 to 3.0 g/10 min, and most preferably0.2 to 1.0 g/10 min determined according to ISO 1133.

The composition preferably has a melt flow rate MFR₂₁ (190° C., 21.6 kg)of 15 to 300 g/10 min, preferably of 20 to 250 g/10 min, and mostpreferably of 25 to 200 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 935.0kg/m³ and equal to or less than 955.0 kg/m³, more preferably of equal toor more than 936.5 kg/m³ and equal to or less than 952.0 kg/m³, and mostpreferably of equal to or more than 938.0 kg/m³ and equal to or lessthan 950.0 kg/m³ determined according to ISO 1183-1:2004.

HD Pipe

For HD pipes the composition according to the present inventionpreferably has a melt flow rate MFR₅ (190° C., 5 kg) of 0.05 to 1.0 g/10min, more preferably of 0.08 to 0.7 g/10 min, and most preferably 0.1 to0.4 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 945.0kg/m³ and equal to or less than 965.0 kg/m³, more preferably of equal toor more than 946.5 kg/m³ and equal to or less than 964.0 kg/m³, and mostpreferably of equal to or more than 948.0 kg/m³ and equal to or lessthan 963.0 kg/m³ determined according to ISO 1183-1:2004.

MD Pipe

For MD pipes the composition according to the present inventionpreferably has a melt flow rate MFR₅ (190° C., 5 kg) of 0.05 to 1.0 g/10min, more preferably of 0.08 to 0.7 g/10 min, and most preferably 0.2 to0.6 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 930.0kg/m³ and equal to or less than 945.0 kg/m³, more preferably of equal toor more than 930.0 kg/m³ and equal to or less than 940.0 kg/m³determined according to ISO 1183-1:2004.

Cable Jacketing

For cable jacketing the composition according to the present inventionpreferably has a melt flow rate MFR₅ (190° C., 5 kg) of 0.1 to 3.0 g/10min, more preferably of 0.2 to 2.5 g/10 min, and most preferably 0.3 to3.0 g/10 min determined according to ISO 1133.

The composition preferably has a density of equal to or more than 918.0kg/m³ and equal to or less than 965.0 kg/m³, more preferably of equal toor more than 920.0 kg/m³ and equal to or less than 962.0 kg/m³, and mostpreferably of equal to or more than 930.0 kg/m³ and equal to or lessthan 960.0 kg/m³ determined according to ISO 1183-1:2004.

EXAMPLES 1. Determination Methods

a) Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR₅ of polyethylene ismeasured at a temperature 190° C. and a load of 5 kg, the MFR₂ ofpolyethylene at a temperature 190° C. and a load of 2.16 kg and theMFR₂₁ of polyethylene is measured at a temperature of 190° C. and a loadof 21.6 kg. The quantity FRR (flow rate ratio) denotes the ratio of flowrates at different loads. Thus, FRR_(21/5) denotes the value ofMFR₂₁/MFR₅.

a) Density

Density of the polymer was measured according to ISO 1183-1:2004 MethodA on compression moulded specimen prepared according to EN ISO 1872-2(February 2007) and is given in kg/m³.

b) Comonomer Content

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used toquantify the comonomer content of the polymers.

Quantitative ¹³C{¹H} NMR spectra recorded in the molten-state using aBruker Advance III 500 NMR spectrometer operating at 500.13 and 125.76MHz for ¹H and ¹³C respectively. All spectra were recorded using a ¹³Coptimised 7 mm magic-angle spinning (MAS) probehead at 150° C. usingnitrogen gas for all pneumatics. Approximately 200 mg of material waspacked into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz.This setup was chosen primarily for the high sensitivity needed forrapid identification and accurate quantification. {[1], [2], [6]}Standard single-pulse excitation was employed utilising the transientNOE at short recycle delays of 3 s {[1], [3]} and the RS-HEPT decouplingscheme {[4], [5]}. A total of 1024 (1k) transients were acquired perspectrum. This setup was chosen due its high sensitivity towards lowcomonomer contents.

Quantitative ¹³C{¹H} NMR spectra were processed, integrated andquantitative properties determined using custom spectral analysisautomation programs. All chemical shifts are internally referenced tothe bulk methylene signal (δ+) at 30.00 ppm {[9]}.

Characteristic signals corresponding to the incorporation of 1-hexenewere observed {[9]} and all contents calculated with respect to allother monomers present in the polymer.

H=I· _(B4)

With no other signals indicative of other comonomer sequences, i.e.consecutive comonomer incorporation, observed the total 1-hexenecomonomer content was calculated based solely on the amount of isolated1-hexene sequences:

H _(total) =H

Characteristic signals resulting from saturated end-groups wereobserved. The content of such saturated end-groups was quantified usingthe average of the integral of the signals at 22.84 and 32.23 ppmassigned to the 2s and 2s sites respectively:

S=(1/2)*(I _(2S) +I _(3S))

The relative content of ethylene was quantified using the integral ofthe bulk methylene (δ+) signals at 30.00 ppm:

E=(1/2)*I _(δ+)

The total ethylene comonomer content was calculated based the bulkmethylene signals and accounting for ethylene units present in otherobserved comonomer sequences or end-groups:

E _(total) =E+(5/2)*B+(3/2)*S

The total mole fraction of 1-hexene in the polymer was then calculatedas:

fH=(H _(total)(E _(total) H _(total))

The total comonomer incorporation of 1-hexene in mole percent wascalculated from the mole fraction in the usual manner:

H[mol %]=100*fH

The total comonomer incorporation of 1-hexene in weight percent wascalculated from the mole fraction in the standard manner:

H[wt %]=100*(fH*84.16)/((fH*84.16)+((1−fH)*28.05))

-   [1] Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.    W., Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382.-   [2] Parkinson, M., Klimke, K., Spiess, H. W., Wilhelm, M., Macromol.    Chem. Phys. 2007; 208:2128.-   [3] Pollard, M., Klimke, K., Graf, R., Spiess, H. W., Wilhelm, M.,    Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004; 37:813.-   [4] Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239-   [5] Griffin, J. M., Tripon, C., Samoson, A., Filip, C., and    Brown, S. P., Mag. Res. in Chem. 2007 45, Si, S198-   [6] Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M.,    Gaborieau, M., Polymer 50 (2009) 2373-   [7] Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha,    A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225-   [8] Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R.,    Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128-   [9] J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989,    C29, 201.

c) Rheological Parameters

The characterization of polymer melts by dynamic shear measurementscomplies with ISO standards 6721-1 and 6721-10. The measurements wereperformed on an ARES (TA Instruments) stress controlled rotationalrheometer, equipped with a 25 mm parallel plate geometry. Measurementswere undertaken on compression moulded plates using nitrogen atmosphereand setting a strain within the linear viscoelastic regime. Theoscillatory shear tests were done at 230° C. applying a frequency rangebetween 0.0631 and 100 rad/s and setting a gap of 2.0 mm.

In a dynamic shear experiment the probe is subjected to a homogeneousdeformation at a sinusoidal varying shear strain or shear stress (strainand stress controlled mode, respectively). On a controlled strainexperiment, the probe is subjected to a sinusoidal strain that can beexpressed by

γ(t)=γ₀ sin(ωt)  (1)

If the applied strain is within the linear viscoelastic regime, theresulting sinusoidal stress response can be given by

σ(t)=σ₀ sin(ωt+δ)  (2)

where σ₀, and γ₀ are the stress and strain amplitudes, respectively; ωis the angular frequency; δ is the phase shift (loss angle betweenapplied strain and stress response); t is the time.

Dynamic test results are typically expressed by means of severaldifferent rheological functions, namely the shear storage modulus, G′,the shear loss modulus, G″, the complex shear modulus, G*, the complexshear

viscosity, η*, the dynamic shear viscosity, η′, the out-of-phasecomponent of the complex shear viscosity, η″ and the loss tangent, tanq, which can be expressed as follows:

$\begin{matrix}{G^{\prime} = {\frac{\sigma_{0}}{\gamma_{0}}\cos \; {\delta \lbrack{Pa}\rbrack}}} & (3) \\{G^{''} = {\frac{\sigma_{0}}{\gamma_{0}}\sin \; {\delta \lbrack{Pa}\rbrack}}} & (4) \\{G^{*} = {G^{\prime} + {{iG}^{''}\lbrack{Pa}\rbrack}}} & (5) \\{\eta^{*} = {\eta^{\prime} - {i\; {\eta^{''}\lbrack {{Pa} \cdot s} \rbrack}}}} & (6) \\{\eta^{\prime} = {\frac{G^{''}}{\omega}\lbrack {{Pa} \cdot s} \rbrack}} & (7) \\{\eta^{''} = {\frac{G^{\prime}}{\omega}\lbrack {{Pa} \cdot s} \rbrack}} & (8)\end{matrix}$

The determination of so-called Shear Thinning Index, which correlateswith MWD and is independent of Mw, is done as described in equation 9.

SHI(x/y)=η*_(ω=x rad/s)/η*_(ω=y rad/s)  (9)

For example, the SHI_((0.1/100)) is defined as the ratio of the complexviscosity determined at a frequency of 0.1 rad/s to the complexviscosity determined at a frequency of 100 rad/s.

The values of storage modulus (G′), loss modulus (G″), complex modulus(G*) and complex viscosity (η*) were obtained as a function of frequency(ω).

Thereby, e.g. η*₃₀₀ (eta*₃₀₀) is used as abbreviation for the complexviscosity at the frequency of 300 rad/s and η_(0.05) (eta*_(0.05)) isused as abbreviation for the complex viscosity at the frequency of 0.05rad/s.

The values are determined by means of a single point interpolationprocedure, as defined by Rheoplus software. In interpolation the optionfrom Rheoplus “Interpolate y-values to x-values from parameter” and the“logarithmic interpolation type” were applied.

REFERENCES

-   [1] Rheological characterization of polyethylene fractions”    Heino, E. L., Lehtinen, A., Tanner J., Seppälä, J., Neste Oy,    Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th    (1992), 1, 360-362-   [2] The influence of molecular structure on some rheological    properties of polyethylene”, Heino, E. L., Borealis Polymers Oy,    Porvoo, Finland, Annual Transactions of the Nordic Rheology Society,    1995.).-   [3] Definition of terms relating to the non-ultimate mechanical    properties of polymers, Pure & Appl. Chem., Vol. 70, No. 3, pp.    701-754, 1998.

d) Molecular Weight

Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution(MWD) and its broadness, described by polydispersity index, PDI=Mw/Mn(wherein Mn is the number average molecular weight and Mw is the weightaverage molecular weight) were determined by Gel PermeationChromatography (GPC) according to ISO 16014-4:2003 and ASTM D 6474-99using the following formulas:

$\begin{matrix}{M_{n} = \frac{\sum\limits_{i = 1}^{N}A_{i}}{\sum( {A_{i}/M_{i}} )}} & (1) \\{M_{w} = \frac{\sum\limits_{i = 1}^{N}( {A_{i} \times M_{i}} )}{\sum A_{i}}} & (2) \\{M_{z} = \frac{\sum\limits_{i = 1}^{N}( {A_{i} \times M_{i}^{2}} )}{\sum( {A_{i}/M_{i}} )}} & (3)\end{matrix}$

For a constant elution volume interval ΔV_(i), where A_(i) and M_(i) arethe chromatographic peak slice area and polyolefin molecular weight(MW).

A PolymerChar GPC instrument, equipped with infrared (IR) detector wasused with 3×Olexis and 1× Olexis Guard columns from Polymer Laboratoriesand 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tertbutyl-4-methyl-phenol) as solvent at 160° C. and at a constant flow rateof 1 mL/min. 200 μL of sample solution were injected per analysis. Thecolumn set was calibrated using universal calibration (according to ISO16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards inthe range of 0.5 kg/mol to 11 500 kg/mol. Mark Houwink constants for PS,PE and PP used are as described per ASTM D 6474-99. All samples wereprepared by dissolving 5.0-9.0 mg of polymer in 8 mL (at 160° C.) ofstabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hoursfor PE at 160° C. under continuous gentle shaking in the autosampler ofthe GPC instrument.

e) White Spot Area (WSA) and ISO Rating

A sample of the composition (including a pigment to make theinhomogeneities visible, i.e. carbon black added to Reference Example 1in an amount as listed below) which is obtained after the compoundingsteps as described for the different examples below, is analysed byfirstly obtaining 6 microtome cuts of 6 different parts of the sample(thickness about 10 micrometer, diameter 3 to 5 mm).

Microtome cuts with a thickness of about 10 μm were made from 6 pelletsof the respective sample perpendicular to extrusion direction. Themicrotome cuts were characterized by light microscopy (ZEISS microscopeAxioimager) at a magnification of 100 quantitatively using the WhiteSpot Area method with SCANDIUM software. In this method all detectedareas of white particles in all cuts of every sample were summarized andrelated to the total area of investigation (1.63 mm²). The investigatedarea of each cut was chosen on a random basis.

The same images used for WSA were also evaluated by the rating followingISO 18553.

f) Viscosity Average Molecular Weight of the UHMWPE

The viscosity average molecular weight of the ultra-high molecularweight polyethylene was determined according to ASTM 4020-81.

2. Figures

FIGS. 1 and 2 show the screw design of the extruders used for theproduction of the polymer mixtures and the polymers of the examplesaccording to the invention.

FIG. 1: Coperion ZSK 26 screw design for the production of the firstpolyethylene resin (A) or the mixture of the polyethylene resins (A) and(C) melt

-   -   Raw material feed at the arrow on the top of the figure; melt        exit at the right

FIG. 2: Coperion ZSK 40 screw design for the production of the polymersof the examples according to the invention

-   -   Pellets were fed at the left arrow, the melt of the first        polyethylene resin (A) or the mixture of the polyethylene        resins (A) and (C) from the ZSK 26 was fed at the right arrow;        melt exit at the right

FIG. 3: Leistritz ZE 27 MAXX screw design used in the ComparativeExample 1. Polymer mixture was fed at the arrow on the right and themelt exits at the left.

3. Examples a) Polymerization of Reference Example 1 (RE1)

A loop reactor having a volume of 50 dm³ was operated continuously at atemperature of 60° C. and a pressure of 62 bar. Into the reactor wereintroduced 42 kg/h of propane diluent, 2 kg/h of ethylene and 35 g/h ofhydrogen. In addition 6.3 g/h of a solid polymerization catalystcomponent sold by BASF under a trade name of Lynx 200 was introducedinto the reactor together with triethylaluminium cocatalyst so that theratio of aluminium to titanium was 30 mol/mol. The rate of polymerproduction was about 1.8 kg/h.

The slurry from the 50 dm³ loop reactor was withdrawn and transferredcontinuously to another loop reactor having a volume of 500 dm³ andwhich was operated at a temperature of 95° C. and a pressure of 60 bar.Into the reactor were introduced additional propane diluent, ethyleneand hydrogen. The ethylene concentration in the fluid mixture was 3.4mol-%, based on the total number of moles in the fluid mixture, themolar ratio of hydrogen to ethylene 650 mol/kmol. The rate of polymerproduction was 32 kg/h and the MFR₂ of the ethylene homopolymer was 500g/10 min.

The slurry from the loop reactor was withdrawn by using settling legsinto a flash vessel operated at a temperature of 50° C. and a pressureof 3 bar where the hydrogen and major part of the hydrocarbons wasremoved from the polymer. The ethylene homopolymer was directed into afluidized bed gas phase reactor operated at 85° C. temperature and 20bar pressure. Into the reactor were introduced additional ethylene,1-butene comonomer, hydrogen and nitrogen as inert gas. The ethyleneconcentration was 11 mol-%, based on the total number of moles in thegas mixture. Hydrogen and 1-butene were added so that the bimodalpolymer had a density of 943 kg/m³, an MFR₅ of 2 g/10 min and a weightaverage molecular weight Mw of 129 kg/mol. The split between the polymerproduced in the loop reactor and the gas phase reactor was 50/50.

The resulting polymer powder was dried from hydrocarbons and mixed with3000 ppm of Irganox B225, 1000 ppm of calcium stearate and 2.4% ofcarbon black, based on the final composition. A part of the mixture wasthen extruded into pellets by using a CIM90P twin screw extruder(manufactured by Japan Steel Works). The throughput was 200 kg/h and thespecific energy input 170 kWh/ton.

b) Production of the Examples According to the Invention InventiveExample 1 (Ex1)

An ultra high molecular weight PE (UHMWPE-GC002, supplied by JingchemCorporation, Beijing, China, and having a viscosity average molecularweight of 1 650 000 g/mol, a density of 0.934 g/cm³, a bulk density of0.42 g/cm³, and a content of volatile matter of 0.12% by weight wasmixed with Irganox B225 and calcium stearate so that the powder mixturecontained 0.3 parts per hundred of B225 and 0.15 parts per hundred ofcalcium stearate. This powder mixture was separately dosed into thehopper of a Coperion ZSK 26 twin screw extruder (screw diameter 25.3 mm;L/D 16) together with also separately dosed bimodal polymer pelletsproduced according to the Reference Example 1 above so that final blendcontained 50% by weight of the UHMWPE—additive mixture and 50% by weightof the pellets of Reference Example 1. Both components were extruded byusing the Coperion ZSK 26 twin screw extruder with a screw designaccording to FIG. 1. The adjusted barrel temperature during theextrusion was 170° C. and the screw speed was 500 min⁻¹. The throughputwas 10 kg/h.

The pellets of Reference Example 1 were introduced into the hopper of aCoperion ZSK 40 twin screw extruder with a screw design according toFIG. 2. An increasing temperature program from 200° C. to 220° C. wasadjusted, the throughput was 15 kg/h and a screw speed of 150 min⁻¹ waschosen. The melt from the ZSK 26 extruder was introduced into the meltport of ZSK 40. At the end of the extruder the mixed melt streams werepassed through a die plate, cooled in a water-bath and cut to pellets.The pellets were then dried and recovered. From the pellets thedispersion (as the white spot area) and the dynamic viscosity wasmeasured. Table 1 shows the WSA, ISO rating, η_(0.1), η₁₀₀,SHI_(0.1/100), MFR₅ and density.

Inventive Example 2 (Ex2)

The procedure of Inventive Example 1 (Ex1) was repeated except that theadjusted barrel temperature in ZSK 26 was 200° C. Table 1 shows the WSA,ISO rating, η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

Inventive Example 3 (Ex3)

The procedure of Inventive Example 2 (Ex2) was repeated except that thefeed into the ZSK 26 contained 70% by weight of the UHMWPE—additivemixture and 30% of the pellets of Reference Example 1. Table 1 shows theWSA, ISO rating, η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

Inventive Example 4 (Ex4)

The procedure of Inventive Example 1 (Ex1) was repeated except that thethroughput was 40 kg/h in ZSK 40. Table 2 shows the WSA, ISO rating,η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

Inventive Example 5 (Ex5)

The procedure of Inventive Example 4 (Ex4) was repeated except that theadjusted barrel temperature in ZSK 26 was 200° C. Table 2 shows the WSA,ISO rating, η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

Inventive Example 6 (Ex6)

The procedure of Inventive Example 5 (Ex5) was repeated except that thefeed into the ZSK 26 contained 70% by weight of the UHMWPE—additivemixture and 30% of the pellets of Reference Example 1 and that thethroughput in ZSK 40 was 60 kg/h. Table 2 shows the WSA, ISO rating,η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

Comparative Example 1 (CE1)

An ultra-high molecular weight PE (UHMWPE-GC002, supplied by JingchemCorporation, Beijing, China, and having a viscosity average molecularweight of 1 650 000 g/mol, a density of 0.934 g/cm³, a bulk density of0.42 g/cm³, and a content of volatile matter of 0.12% by weight wasmixed with Irganox B225 and calcium stearate so that the powder mixturecontained 0.3 parts per hundred of B225 and 0.15 parts per hundred ofcalcium stearate. This powder mixture was separately dosed into thehopper of a Leistritz ZSE 27 MAXX extruder together with also separatelydosed bimodal polymer pellets produced according to the ReferenceExample 1 above so that final blend contained 10% by weight of theUHMWPE—additive mixture and 90% by weight of the pellets of ReferenceExample 1. The polymer mixture was extruded by using a Leistritz ZE 27MAXX twin screw extruder (screw diameter 28.3 mm; L/D 36) with a screwdesign according to FIG. 3. The adjusted barrel temperature during theextrusion was 200° C. to 220° C. and the screw speed was 500 min⁻¹. Thethroughput was 10 kg/h.

At the end of the extruder the melt was passed through a die plate,cooled in a water-bath and cut to pellets. The pellets were then driedand recovered. From the pellets the dispersion (as the white spot area)and the dynamic viscosity was measured. Table 1 shows the WSA, ISOrating, η_(0.1), η₁₀₀, SHI_(0.1/100), MFR₅ and density.

c) Results

TABLE 1 Example Ex1 Ex2 Ex3 RE1 CE1 Amount UHMW- PE 20 20 28 0 10 [wt %]ISO rating 3.1 2.1 1.3 n.d. 4.4 WSA [%] 0.83 0.20 0.04 n.d. 5.28 η_(0.1)[Pas] 43070 27450 14700 12450 16620 η₁₀₀ [Pas] 2090 1800 1510 1060 1295SHI_(0.1/100) 20.6 15.3 9.7 11.7 12.8 η_(0.1)(Ex)/η_(0.1)(RE1) 3.46 2.201.18 — 1.33 MFR₅ [g/10 min] 0.51 0.88 1.71 n.d. 1.32 Density [kg/m³] 948947 944 n.d. 945 n.d. not determined

TABLE 2 Example Ex4 Ex5 Ex6 RE1 CE1 Amount UHMW- PE 10 10 14 0 10 [wt %]ISO rating 2.4 3.9 2.8 n.d. 4.4 WSA [%] 0.24 2.69 0.51 n.d. 5.28 η_(0.1)[Pas] 12830 15580 13020 12450 16620 η₁₀₀ [Pas] 1200 1330 1130 1060 1295SHI_(0.1/100) 10.7 11.7 11.5 11.7 12.8 η_(0.1)(Ex)/η_(0.1)(RE1) 1.031.25 1.05 — 1.33 MFR₅ [g/10 min] 2.04 1.56 1.85 n.d. 1.32 Density[kg/m³] 947 946 948 n.d. 945 n.d. not determined

1. A process for producing a multimodal polyethylene compositioncomprising the following steps: i) at least partially melting a firstpolyethylene resin (A) having a viscosity average molecular weight My ofequal to or more than 700 kg/mol to equal to or less than 10,000 kg/moland a density of equal to or more than 920 kg/m³ to equal to or lessthan 960 kg/m³ in a first homogenizing device, ii) at least partiallymelting a second polyethylene resin (B) having a Mw of equal to or morethan 50 kg/mol to less than 700 kg/mol, and a density of equal to ormore than 910 kg/m³ to equal to or less than 960 kg/m³ in a secondhomogenizing device, iii) combining the at least partially molten firstpolyethylene resin (A) with the at least partially molten secondpolyethylene resin (B) in said second homogenizing device, iv)compounding the combined first polyethylene resin (A) and secondpolyethylene resin (B) in said second homogenizing device to form amultimodal polyethylene composition, wherein the multimodal polyethylenecomposition has a melt flow rate MFR₅ (190° C., 5 kg) of 0.01 to 10.0g/10 min and a density of equal to or more than 910 kg/m³ to equal to orless than 970 kg/m³.
 2. The process according to claim 1, wherein priorto the melting step i) the first polyethylene resin (A) is blended witha third polyethylene resin (C) and the blend of the first polyethyleneresin (A) and the third polyethylene resin (C) is fed to the firsthomogenization device.
 3. The process according to claim 2, wherein theweight ratio of the first polyethylene resin (A) to the thirdpolyethylene resin (C) in the blend of polyethylene resins (A) and (C)is from 45:55 to 80:20.
 4. The process according to claim 2, wherein thefirst polyethylene resin (A) is completely molten before combining withthe second polyethylene resin (B).
 5. The process according to claim 1,said second homogenizing device is the main extruder for compounding themultimodal polyethylene composition.
 6. The process according claim 5,wherein said first homogenizing device is a side feed extruder having anexit which is connected to downstream of the melting zone of the mainextruder.
 7. The process according to claim 1, wherein the weight ratioof the first polyethylene resin (A) to the second polyethylene resin (B)in the polyethylene composition is 0.5:99.5 to 50:50.
 8. The processaccording to claim 1, wherein the second polyethylene resin (B) is amultimodal polyethylene resin.
 9. A polyethylene composition obtainableby a process comprising the following steps: i) at least partiallymelting a first polyethylene resin (A) having a viscosity averagemolecular weight My of equal to or more than 700 kg/mol to equal to orless than 10,000 kg/mol and a density of equal to or more than 920 kg/m³to equal to or less than 960 kg/m³ in a first homogenizing device, ii)at least partially melting a second polyethylene resin (B) having a Mwof equal to or more than 50 kg/mol to less than 700 kg/mol, and adensity of equal to or more than 910 kg/m³ to equal to or less than 960kg/m³ in a second homogenizing device, iii) combining the at leastpartially molten first polyethylene resin (A) with the at leastpartially molten second polyethylene resin (B) in said secondhomogenizing device, iv) compounding the combined first polyethyleneresin (A) and second polyethylene resin (B) in said second homogenizingdevice to form a multimodal polyethylene composition, wherein themultimodal polyethylene composition has a melt flow rate MFR₅ (190° C.,5 kg) of 0.01 to 10.0 g/10 min and a density of equal to or more than910 kg/m³ to equal to or less than 970 kg/m³.
 10. The polyethylenecomposition according to claim 9 wherein prior to the melting step i)the first polyethylene resin (A) is blended with a third polyethyleneresin (C) and the blend of the first polyethylene resin and the thirdpolyethylene resin (C) is fed to the first homogenization device. 11.The polyethylene composition according to claim 10, wherein the weightratio of the first polyethylene resin (A) to the third polyethyleneresin (C) in the blend of polyethylene resins (A) and (C) is from 45:55to 80:20.
 12. The polyethylene composition according to claim 9 having awhite spot area of not more than 5.0%.
 13. The polyethylene compositionaccording to claim 9 having a shear thinning index SHI_(0.1/100), beingthe ratio of the complex viscosities at the shear rates of 0.1 Pa and100 Pa, of 7.0 to
 30. 14. The polyethylene composition according toclaim 9, wherein the ratio of the complex viscosity at a shear rate of0.1 Pa of the multimodal polyethylene composition,eta_(0.1)(Composition), to the complex viscosity determined for thecomplex shear modulus of 0.1 kPa of the polyethylene resin (B),eta_(0.1) (B), is in the range of 0.8 to 5.0.
 15. The polyethylenecomposition according to claim 9, wherein the amount of polyethyleneresin (A) is 0.5 to 50 wt % of the total polyethylene composition. 16.The process according to claim 3, wherein the first polyethylene resin(A) is completely molten before combining with the second polyethyleneresin (B).
 17. The process according to claim 3, wherein the secondpolyethylene resin (B) is a multimodal polyethylene resin.
 18. Theprocess according to claim 7, wherein the second polyethylene resin (B)is a multimodal polyethylene resin.